Chemistry – Acids, bases and salts - Oxides | e-Consult

The reactivity of an oxide with water is directly related to the electronegativity difference between the metal/non-metal and oxygen. Oxides formed from metals and non-metals with a significant electronegativity difference are generally more reactive with water than oxides formed from elements with similar electronegativities. This is because the resulting oxide ions (either positive or negative) have a stronger affinity for water molecules.

For example, sodium oxide (Na₂O) is highly reactive with water. Sodium is a metal and oxygen is a non-metal, resulting in a large electronegativity difference. Na₂O readily reacts with water to produce sodium hydroxide (NaOH) and heat. This is because the strong positive charge of the sodium ion and the negative charge of the oxide ion attract water molecules strongly, leading to a rapid reaction.

Conversely, magnesium oxide (MgO) is less reactive with water than sodium oxide. Magnesium is a metal and oxygen is a non-metal, but the electronegativity difference is smaller than that between sodium and oxygen. MgO reacts with water to produce magnesium hydroxide (Mg(OH)₂), but the reaction is slower and less vigorous than the reaction of Na₂O with water. The weaker attraction between the magnesium ion and water molecules results in a slower reaction.

Another example is carbon dioxide (CO₂). Carbon is a non-metal and oxygen is a non-metal, so the electronegativity difference is relatively small. CO₂ is much less reactive with water than Na₂O. The weaker interaction between the carbon and oxygen atoms in CO₂ and the weaker attraction between the carbon/oxygen ions and water molecules result in a slow reaction with water, forming carbonic acid (H₂CO₃).